Basically a wrapper class to hold both the branch predictor and the BTB. More...

#include <bpred_unit.hh>

Inheritance diagram for gem5::branch_prediction::BPredUnit:

Classes
struct	BPredUnitStats
	Statistics. More...

struct	PredictorHistory

Public Member Functions
	BPredUnit (const Params &p)
	Branch Predictor Unit (BPU) interface functions.

void	regProbePoints () override
	Register probe points for this object.

void	drainSanityCheck () const
	Perform sanity checks after a drain.

bool	predict (const StaticInstPtr &inst, const InstSeqNum &seqNum, PCStateBase &pc, ThreadID tid)
	Predicts whether or not the instruction is a taken branch, and the target of the branch if it is taken.

void	update (const InstSeqNum &done_sn, ThreadID tid)
	Tells the branch predictor to commit any updates until the given sequence number.

void	squash (const InstSeqNum &squashed_sn, ThreadID tid)
	Squashes all outstanding updates until a given sequence number.

void	squash (const InstSeqNum &squashed_sn, const PCStateBase &corr_target, bool actually_taken, ThreadID tid, bool from_commit=true)
	Squashes all outstanding updates until a given sequence number, and corrects that sn's update with the proper address and taken/not taken.

Public Member Functions inherited from gem5::SimObject
const Params &	params () const

	SimObject (const Params &p)

virtual	~SimObject ()

virtual void	init ()
	init() is called after all C++ SimObjects have been created and all ports are connected.

virtual void	loadState (CheckpointIn &cp)
	loadState() is called on each SimObject when restoring from a checkpoint.

virtual void	initState ()
	initState() is called on each SimObject when not restoring from a checkpoint.

virtual void	regProbeListeners ()
	Register probe listeners for this object.

ProbeManager *	getProbeManager ()
	Get the probe manager for this object.

virtual Port &	getPort (const std::string &if_name, PortID idx=InvalidPortID)
	Get a port with a given name and index.

virtual void	startup ()
	startup() is the final initialization call before simulation.

DrainState	drain () override
	Provide a default implementation of the drain interface for objects that don't need draining.

virtual void	memWriteback ()
	Write back dirty buffers to memory using functional writes.

virtual void	memInvalidate ()
	Invalidate the contents of memory buffers.

void	serialize (CheckpointOut &cp) const override
	Serialize an object.

void	unserialize (CheckpointIn &cp) override
	Unserialize an object.

Public Member Functions inherited from gem5::EventManager
EventQueue *	eventQueue () const

void	schedule (Event &event, Tick when)

void	deschedule (Event &event)

void	reschedule (Event &event, Tick when, bool always=false)

void	schedule (Event *event, Tick when)

void	deschedule (Event *event)

void	reschedule (Event *event, Tick when, bool always=false)

void	wakeupEventQueue (Tick when=(Tick) -1)
	This function is not needed by the usual gem5 event loop but may be necessary in derived EventQueues which host gem5 on other schedulers.

void	setCurTick (Tick newVal)

	EventManager (EventManager &em)
	Event manger manages events in the event queue.

	EventManager (EventManager *em)

	EventManager (EventQueue *eq)

Public Member Functions inherited from gem5::Serializable
	Serializable ()

virtual	~Serializable ()

void	serializeSection (CheckpointOut &cp, const char *name) const
	Serialize an object into a new section.

void	serializeSection (CheckpointOut &cp, const std::string &name) const

void	unserializeSection (CheckpointIn &cp, const char *name)
	Unserialize an a child object.

void	unserializeSection (CheckpointIn &cp, const std::string &name)

Public Member Functions inherited from gem5::Drainable
DrainState	drainState () const
	Return the current drain state of an object.

virtual void	notifyFork ()
	Notify a child process of a fork.

Public Member Functions inherited from gem5::statistics::Group
	Group (Group parent, const char name=nullptr)
	Construct a new statistics group.

virtual	~Group ()

virtual void	regStats ()
	Callback to set stat parameters.

virtual void	resetStats ()
	Callback to reset stats.

virtual void	preDumpStats ()
	Callback before stats are dumped.

void	addStat (statistics::Info *info)
	Register a stat with this group.

const std::map< std::string, Group * > &	getStatGroups () const
	Get all child groups associated with this object.

const std::vector< Info * > &	getStats () const
	Get all stats associated with this object.

void	addStatGroup (const char name, Group block)
	Add a stat block as a child of this block.

const Info *	resolveStat (std::string name) const
	Resolve a stat by its name within this group.

void	mergeStatGroup (Group *block)
	Merge the contents (stats & children) of a block to this block.

	Group ()=delete

	Group (const Group &)=delete

Group &	operator= (const Group &)=delete

Public Member Functions inherited from gem5::Named
	Named (const std::string &name_)

virtual	~Named ()=default

virtual std::string	name () const

Protected Member Functions
virtual bool	lookup (ThreadID tid, Addr pc, void *&bp_history)=0
	Looks up a given conditional branch PC of in the BP to see if it is taken or not taken.

virtual void	updateHistories (ThreadID tid, Addr pc, bool uncond, bool taken, Addr target, void *&bp_history)=0
	Ones done with the prediction this function updates the path and global history.

virtual void	squash (ThreadID tid, void *&bp_history)=0

virtual void	update (ThreadID tid, Addr pc, bool taken, void *&bp_history, bool squashed, const StaticInstPtr &inst, Addr target)=0
	Updates the BP with taken/not taken information.

bool	BTBValid (ThreadID tid, Addr instPC)
	Looks up a given PC in the BTB to see if a matching entry exists.

const PCStateBase *	BTBLookup (ThreadID tid, PCStateBase &instPC)
	Looks up a given PC in the BTB to get the predicted target.

const StaticInstPtr	BTBGetInst (ThreadID tid, Addr instPC)
	Looks up a given PC in the BTB to get current static instruction information.

void	BTBUpdate (ThreadID tid, Addr instPC, const PCStateBase &target)
	Updates the BTB with the target of a branch.

void	dump ()

Protected Member Functions inherited from gem5::Drainable
	Drainable ()

virtual	~Drainable ()

virtual void	drainResume ()
	Resume execution after a successful drain.

void	signalDrainDone () const
	Signal that an object is drained.

Protected Attributes
const unsigned	numThreads
	Number of the threads for which the branch history is maintained.

const bool	requiresBTBHit
	Requires the BTB to hit for returns and indirect branches.

const unsigned	instShiftAmt
	Number of bits to shift instructions by for predictor addresses.

std::vector< History >	predHist
	The per-thread predictor history.

BranchTargetBuffer *	btb
	The BTB.

ReturnAddrStack *	ras
	The return address stack.

IndirectPredictor *	iPred
	The indirect target predictor.

gem5::branch_prediction::BPredUnit::BPredUnitStats	stats

Protected Attributes inherited from gem5::SimObject
const SimObjectParams &	_params
	Cached copy of the object parameters.

Protected Attributes inherited from gem5::EventManager
EventQueue *	eventq
	A pointer to this object's event queue.

Private Types
typedef BranchPredictorParams	Params

typedef enums::TargetProvider	TargetProvider

typedef std::deque< PredictorHistory * >	History

Private Member Functions
bool	predict (const StaticInstPtr &inst, const InstSeqNum &seqNum, PCStateBase &pc, ThreadID tid, PredictorHistory *&bpu_history)
	Internal prediction function.

void	squashHistory (ThreadID tid, PredictorHistory *&bpu_history)
	Squashes a particular branch instance.

void	commitBranch (ThreadID tid, PredictorHistory *&bpu_history)
	Commit a particular branch.

PMU Probe points.
probing::PMUUPtr	ppBranches
	Branches seen by the branch predictor.

probing::PMUUPtr	ppMisses
	Miss-predicted branches.

probing::PMUUPtr	pmuProbePoint (const char *name)
	Helper method to instantiate probe points belonging to this object.

Additional Inherited Members
Public Types inherited from gem5::SimObject
typedef SimObjectParams	Params

Static Public Member Functions inherited from gem5::SimObject
static void	serializeAll (const std::string &cpt_dir)
	Create a checkpoint by serializing all SimObjects in the system.

static SimObject *	find (const char *name)
	Find the SimObject with the given name and return a pointer to it.

static void	setSimObjectResolver (SimObjectResolver *resolver)
	There is a single object name resolver, and it is only set when simulation is restoring from checkpoints.

static SimObjectResolver *	getSimObjectResolver ()
	There is a single object name resolver, and it is only set when simulation is restoring from checkpoints.

Static Public Member Functions inherited from gem5::Serializable
static const std::string &	currentSection ()
	Gets the fully-qualified name of the active section.

static void	generateCheckpointOut (const std::string &cpt_dir, std::ofstream &outstream)
	Generate a checkpoint file so that the serialization can be routed to it.

Detailed Description

Basically a wrapper class to hold both the branch predictor and the BTB.

Definition at line 70 of file bpred_unit.hh.

Member Typedef Documentation

◆ History

typedef std::deque<PredictorHistory*> gem5::branch_prediction::BPredUnit::History

private

Definition at line 341 of file bpred_unit.hh.

◆ Params

typedef BranchPredictorParams gem5::branch_prediction::BPredUnit::Params

private

Definition at line 72 of file bpred_unit.hh.

◆ TargetProvider

typedef enums::TargetProvider gem5::branch_prediction::BPredUnit::TargetProvider

private

Definition at line 73 of file bpred_unit.hh.

Constructor & Destructor Documentation

◆ BPredUnit()

gem5::branch_prediction::BPredUnit::BPredUnit ( const Params & p )

Branch Predictor Unit (BPU) interface functions.

Parameters

params The params object, that has the size of the BP and BTB.

Definition at line 58 of file bpred_unit.cc.

Member Function Documentation

◆ BTBGetInst()

const StaticInstPtr gem5::branch_prediction::BPredUnit::BTBGetInst	(	ThreadID	tid,
		Addr	instPC )

inlineprotected

Looks up a given PC in the BTB to get current static instruction information.

This is necessary in a decoupled frontend as the information does not usually exist at that this point. Only for instructions (branches) that hit in the BTB this information is available as the BTB stores them together with the target.

Parameters

inst_PC The PC to look up.

Returns: The static instruction info of the given PC if existant.

Definition at line 227 of file bpred_unit.hh.

References btb, and gem5::branch_prediction::BranchTargetBuffer::getInst().

◆ BTBLookup()

const PCStateBase * gem5::branch_prediction::BPredUnit::BTBLookup	(	ThreadID	tid,
		PCStateBase &	instPC )

inlineprotected

Looks up a given PC in the BTB to get the predicted target.

The PC may be changed or deleted in the future, so it needs to be used immediately, and/or copied for use later.

Parameters

tid	The thread id.
inst_PC	The PC to look up.

Returns: The address of the target of the branch.

Definition at line 211 of file bpred_unit.hh.

References btb, gem5::PCStateBase::instAddr(), and gem5::branch_prediction::BranchTargetBuffer::lookup().

◆ BTBUpdate()

void gem5::branch_prediction::BPredUnit::BTBUpdate	(	ThreadID	tid,
		Addr	instPC,
		const PCStateBase &	target )

inlineprotected

Updates the BTB with the target of a branch.

Parameters

inst_PC	The branch's PC that will be updated.
target_PC	The branch's target that will be added to the BTB.

Definition at line 238 of file bpred_unit.hh.

References btb, gem5::branch_prediction::BPredUnit::BPredUnitStats::BTBUpdates, stats, and gem5::branch_prediction::BranchTargetBuffer::update().

◆ BTBValid()

bool gem5::branch_prediction::BPredUnit::BTBValid	(	ThreadID	tid,
		Addr	instPC )

inlineprotected

Looks up a given PC in the BTB to see if a matching entry exists.

Parameters

tid	The thread id.
inst_PC	The PC to look up.

Returns: Whether the BTB contains the given PC.

Definition at line 197 of file bpred_unit.hh.

References btb, and gem5::branch_prediction::BranchTargetBuffer::valid().

◆ commitBranch()

void gem5::branch_prediction::BPredUnit::commitBranch	(	ThreadID	tid,
		PredictorHistory *&	bpu_history )

private

Commit a particular branch.

Parameters

tid	The thread id.
bpu_history	The history of the branch to be commited.

Definition at line 355 of file bpred_unit.cc.

Referenced by update().

◆ drainSanityCheck()

void gem5::branch_prediction::BPredUnit::drainSanityCheck ( ) const

Perform sanity checks after a drain.

Definition at line 89 of file bpred_unit.cc.

References predHist.

Referenced by gem5::o3::Fetch::drainSanityCheck().

◆ dump()

void gem5::branch_prediction::BPredUnit::dump ( )

protected

Definition at line 609 of file bpred_unit.cc.

References gem5::cprintf(), gem5::ArmISA::i, and predHist.

◆ lookup()

virtual bool gem5::branch_prediction::BPredUnit::lookup	(	ThreadID	tid,
		Addr	pc,
		void *&	bp_history )

protectedpure virtual

Looks up a given conditional branch PC of in the BP to see if it is taken or not taken.

Interface functions to the conditional branch predictor

Parameters

pc	The PC to look up.
bp_history	Pointer that will be set to an object that has the branch predictor state associated with the lookup.

Returns: Whether the branch is taken or not taken.

Implemented in gem5::branch_prediction::BiModeBP, gem5::branch_prediction::LocalBP, gem5::branch_prediction::MultiperspectivePerceptron, gem5::branch_prediction::MultiperspectivePerceptronTAGE, gem5::branch_prediction::TAGE, and gem5::branch_prediction::TournamentBP.

Referenced by predict().

◆ pmuProbePoint()

probing::PMUUPtr gem5::branch_prediction::BPredUnit::pmuProbePoint ( const char * name )

protected

Helper method to instantiate probe points belonging to this object.

Parameters

name	Name of the probe point.

Returns: A unique_ptr to the new probe point.

Definition at line 73 of file bpred_unit.cc.

References gem5::SimObject::getProbeManager(), and gem5::Named::name().

Referenced by regProbePoints().

◆ predict() [1/2]

bool gem5::branch_prediction::BPredUnit::predict	(	const StaticInstPtr &	inst,
		const InstSeqNum &	seqNum,
		PCStateBase &	pc,
		ThreadID	tid )

Predicts whether or not the instruction is a taken branch, and the target of the branch if it is taken.

Parameters

inst	The branch instruction.
PC	The predicted PC is passed back through this parameter.
tid	The thread id.

Returns: Returns if the branch is taken or not.

Perform the prediction.

Push the record into the history buffer

Definition at line 99 of file bpred_unit.cc.

References DPRINTF, gem5::MipsISA::pc, predHist, and predict().

Referenced by gem5::o3::Fetch::lookupAndUpdateNextPC(), predict(), gem5::minor::Fetch2::predictBranch(), and gem5::BaseSimpleCPU::preExecute().

◆ predict() [2/2]

bool gem5::branch_prediction::BPredUnit::predict	(	const StaticInstPtr &	inst,
		const InstSeqNum &	seqNum,
		PCStateBase &	pc,
		ThreadID	tid,
		PredictorHistory *&	bpu_history )

private

Internal prediction function.

Fallthrough

All the target predictors did their job. If there is no target its either not taken or a BTB miss. In that case we just fallthrough.

Speculative history update

Now that the prediction is done the predictor may update its histories speculative. (local and global path). A later squash will revert the history update if needed. The actual prediction tables will updated once we know the correct direction.

Definition at line 121 of file bpred_unit.cc.

◆ regProbePoints()

void gem5::branch_prediction::BPredUnit::regProbePoints ( )

overridevirtual

Register probe points for this object.

No probe points by default, so do nothing in base.

Reimplemented from gem5::SimObject.

Definition at line 82 of file bpred_unit.cc.

References pmuProbePoint(), ppBranches, and ppMisses.

◆ squash() [1/3]

void gem5::branch_prediction::BPredUnit::squash	(	const InstSeqNum &	squashed_sn,
		const PCStateBase &	corr_target,
		bool	actually_taken,
		ThreadID	tid,
		bool	from_commit = true )

Squashes all outstanding updates until a given sequence number, and corrects that sn's update with the proper address and taken/not taken.

Parameters

squashed_sn	The sequence number to squash any younger updates up until.
corr_target	The correct branch target.
actually_taken	The correct branch direction.
tid	The thread id.
from_commit	Indicate whether the squash is comming from commit or from decode. Its optional and used for statistics.

Definition at line 449 of file bpred_unit.cc.

◆ squash() [2/3]

void gem5::branch_prediction::BPredUnit::squash	(	const InstSeqNum &	squashed_sn,
		ThreadID	tid )

Squashes all outstanding updates until a given sequence number.

Parameters

squashed_sn	The sequence number to squash any younger updates up until.
tid	The thread id.

Definition at line 393 of file bpred_unit.cc.

References DPRINTF, predHist, and squashHistory().

Referenced by gem5::BaseSimpleCPU::advancePC(), gem5::o3::Fetch::checkSignalsAndUpdate(), squash(), squashHistory(), and gem5::minor::Fetch2::updateBranchPrediction().

◆ squash() [3/3]

virtual void gem5::branch_prediction::BPredUnit::squash	(	ThreadID	tid,
		void *&	bp_history )

protectedpure virtual

Parameters

tid	The thread id.
bp_history	Pointer to the history object. The predictor will need to update any state and delete the object.

Implemented in gem5::branch_prediction::BiModeBP, gem5::branch_prediction::LocalBP, gem5::branch_prediction::LTAGE, gem5::branch_prediction::MultiperspectivePerceptron, gem5::branch_prediction::MultiperspectivePerceptronTAGE, gem5::branch_prediction::TAGE, and gem5::branch_prediction::TournamentBP.

◆ squashHistory()

void gem5::branch_prediction::BPredUnit::squashHistory	(	ThreadID	tid,
		PredictorHistory *&	bpu_history )

private

Squashes a particular branch instance.

Parameters

tid	The thread id.
bpu_history	The history to be squashed.

Definition at line 419 of file bpred_unit.cc.

Referenced by squash().

◆ update() [1/2]

void gem5::branch_prediction::BPredUnit::update	(	const InstSeqNum &	done_sn,
		ThreadID	tid )

Tells the branch predictor to commit any updates until the given sequence number.

Parameters

done_sn	The sequence number to commit any older updates up until.
tid	The thread id.

Definition at line 335 of file bpred_unit.cc.

References commitBranch(), DPRINTF, and predHist.

Referenced by gem5::BaseSimpleCPU::advancePC(), gem5::o3::Fetch::checkSignalsAndUpdate(), commitBranch(), squash(), and gem5::minor::Fetch2::updateBranchPrediction().

◆ update() [2/2]

virtual void gem5::branch_prediction::BPredUnit::update	(	ThreadID	tid,
		Addr	pc,
		bool	taken,
		void *&	bp_history,
		bool	squashed,
		const StaticInstPtr &	inst,
		Addr	target )

protectedpure virtual

Updates the BP with taken/not taken information.

Parameters

tid	The thread id.
PC	The branch's PC that will be updated.
taken	Whether the branch was taken or not taken.
bp_history	Pointer to the branch predictor state that is associated with the branch lookup that is being updated.
squashed	Set to true when this function is called during a squash operation.
inst	Static instruction information
target	The resolved target of the branch (only needed for squashed branches)

Todo: Make this update flexible enough to handle a global predictor.

Implemented in gem5::branch_prediction::BiModeBP, gem5::branch_prediction::LocalBP, gem5::branch_prediction::LTAGE, gem5::branch_prediction::MultiperspectivePerceptron, gem5::branch_prediction::MultiperspectivePerceptronTAGE, gem5::branch_prediction::TAGE, gem5::branch_prediction::TAGE_SC_L, and gem5::branch_prediction::TournamentBP.

◆ updateHistories()

virtual void gem5::branch_prediction::BPredUnit::updateHistories	(	ThreadID	tid,
		Addr	pc,
		bool	uncond,
		bool	taken,
		Addr	target,
		void *&	bp_history )

protectedpure virtual

Ones done with the prediction this function updates the path and global history.

All branches call this function including unconditional once.

Parameters

tid	The thread id.
PC	The branch's PC that will be updated.
uncond	Wheather or not this branch is an unconditional branch.
taken	Whether or not the branch was taken
target	The final target of branch. Some modern predictors use the target in their history.
bp_history	Pointer that will be set to an object that has the branch predictor state associated with the lookup.

Implemented in gem5::branch_prediction::BiModeBP, gem5::branch_prediction::LocalBP, gem5::branch_prediction::MultiperspectivePerceptron, gem5::branch_prediction::MultiperspectivePerceptronTAGE, gem5::branch_prediction::TAGE, and gem5::branch_prediction::TournamentBP.

Referenced by predict().

Member Data Documentation

◆ btb

BranchTargetBuffer* gem5::branch_prediction::BPredUnit::btb

protected

The BTB.

Definition at line 390 of file bpred_unit.hh.

Referenced by BTBGetInst(), BTBLookup(), BTBUpdate(), BTBValid(), predict(), and squash().

◆ instShiftAmt

const unsigned gem5::branch_prediction::BPredUnit::instShiftAmt

protected

Number of bits to shift instructions by for predictor addresses.

Definition at line 380 of file bpred_unit.hh.

◆ iPred

IndirectPredictor* gem5::branch_prediction::BPredUnit::iPred

protected

The indirect target predictor.

Definition at line 396 of file bpred_unit.hh.

Referenced by commitBranch(), predict(), squash(), and squashHistory().

◆ numThreads

const unsigned gem5::branch_prediction::BPredUnit::numThreads

protected

Number of the threads for which the branch history is maintained.

Definition at line 369 of file bpred_unit.hh.

Referenced by gem5::branch_prediction::BPredUnit::BPredUnitStats::BPredUnitStats().

◆ ppBranches

probing::PMUUPtr gem5::branch_prediction::BPredUnit::ppBranches

protected

Branches seen by the branch predictor.

Note: This counter includes speculative branches.

Definition at line 460 of file bpred_unit.hh.

Referenced by predict(), and regProbePoints().

◆ ppMisses

probing::PMUUPtr gem5::branch_prediction::BPredUnit::ppMisses

protected

Miss-predicted branches.

Definition at line 463 of file bpred_unit.hh.

Referenced by regProbePoints(), and squash().

◆ predHist

std::vector<History> gem5::branch_prediction::BPredUnit::predHist

protected

The per-thread predictor history.

This is used to update the predictor as instructions are committed, or restore it to the proper state after a squash.

Definition at line 387 of file bpred_unit.hh.

Referenced by drainSanityCheck(), dump(), predict(), squash(), squash(), and update().

◆ ras

ReturnAddrStack* gem5::branch_prediction::BPredUnit::ras

protected

The return address stack.

Definition at line 393 of file bpred_unit.hh.

Referenced by commitBranch(), predict(), squash(), and squashHistory().

◆ requiresBTBHit

const bool gem5::branch_prediction::BPredUnit::requiresBTBHit

protected

Requires the BTB to hit for returns and indirect branches.

For an advanced front-end there is no other way than a BTB hit to know that the branch exists in the first place. Furthermore, the BPU needs to know the branch type to make the correct RAS operations. This info is only available from the BTB. Low-end CPUs predecoding might be used to identify branches.

Definition at line 377 of file bpred_unit.hh.

Referenced by predict(), and squash().

◆ stats

gem5::branch_prediction::BPredUnit::BPredUnitStats gem5::branch_prediction::BPredUnit::stats

protected

Referenced by BTBUpdate(), commitBranch(), predict(), squash(), and squashHistory().

The documentation for this class was generated from the following files:

cpu/pred/bpred_unit.hh
cpu/pred/bpred_unit.cc

Classes

Public Member Functions

Protected Member Functions

Protected Attributes

Private Types

Private Member Functions

PMU Probe points.

Additional Inherited Members

Detailed Description

Member Typedef Documentation

◆ History

◆ Params

◆ TargetProvider

Constructor & Destructor Documentation

◆ BPredUnit()

Member Function Documentation

◆ BTBGetInst()

◆ BTBLookup()

◆ BTBUpdate()

◆ BTBValid()

◆ commitBranch()

◆ drainSanityCheck()

◆ dump()

◆ lookup()

◆ pmuProbePoint()

◆ predict() [1/2]

◆ predict() [2/2]

Fallthrough

Speculative history update

◆ regProbePoints()

◆ squash() [1/3]

◆ squash() [2/3]

◆ squash() [3/3]

◆ squashHistory()

◆ update() [1/2]

◆ update() [2/2]

◆ updateHistories()

Member Data Documentation

◆ btb

◆ instShiftAmt

◆ iPred

◆ numThreads

◆ ppBranches

◆ ppMisses

◆ predHist

◆ ras

◆ requiresBTBHit

◆ stats